1. Field of the Invention
The invention relates to a flow forming member for a polymer melt.
2. Description of the Prior Art
A flow forming member of this kind is, for example, the outlet point of the injection unit of an injection molding machine, which is formed as a nozzle and which serves for the production of plastic parts. The nozzle, namely a closure nozzle, comprises a rod-like body, namely the closure needle, and a tubular channel along the axis of which the needle can be moved. A polymer melt is pressed under high pressure from an entry point through the ring channel between the channel wall and the needle surface to the nozzle opening. The ring channel must be about six to eight times longer than the tube diameter in order that inhomogeneities of the flow profile at the entry point can even themselves out prior to the outlet point. A reduction of the inhomogeneities is required in order to obtain a desired quality of the product.
In many polymers an irreversible change takes place in the molten state, a thermal degradation whose extent depends on the length of time during which the polymer is molten. Such changes lead to traces in the products which are detrimental to their quality. When the polymer flows through the narrow ring channel of the injection molding nozzle, boundary layers form on the channel wall as well as on the needle surface in which the dwell time of the polymer is substantially longer than in the central region of the ring channel. Therefore, an exacerbated degradation arises in the boundary layers which has a particularly disadvantageous effect on the product quality.
Comparable conditions are present in hollow body extrusion plants for the production of, for example, bottles or tubes. Plants of this kind contain a flow forming member in which a tubular channel likewise connects an inlet point to an outlet point, with the channel containing a rod-like body arranged along the channel axis. The rod-like body is the shaft of a mandrel by means of which a string-like melt flow is reformed into a flow with a ring shaped cross-section.